This invention relates generally to ultrasonic examination of opaque materials, and more particularly to small ultrasonic beams which are both focusable and scannable so as to be suitable for detecting small defects.
An important application of ultrasonics is examining opaque materials and tissue for internal flaws and abnormalities. The examination of materials for flaw detection as well as the examination of tissue for medical diagnostic purposes are improved by the use of small acoustic beams which can be scanned and focused. Such small acoustic beams are desirable in order to detect small defects.
Most of the current methods of ultrasonic examination make use of the well-known Bragg-diffraction effect. In these devices and systems an unfocused, non-scanning ultrasonic beam is propagated in a medium which contains an article or sample to be examined. A light beam is then directed through the medium which is usually transparent. As the light beam traverses the medium, it is diffracted by the ultrasonic waves which have been reflected or scattered by the sample. The resulting diffracted light beam can then be processed and analyzed to render a visual image of the interior of the article examined.
A major drawback to these methods is that the ultrasonic beam is not focusable or scannable, thus small defects are difficult, if not impossible to detect.
Other types of systems using only ultrasonic beams require either mechanical movement of a single element transducer or use of an array transducer to provide scanning of the ultrasonic beam. The single element transducer, however, is usually slow and cumbersome, and does not readily provide for varying the focus of the ultrasonic beam. The array transducer, while it provides some focusing of the ultrasonic beam, can be cumbersome and very expensive.
More recently, small acoustic beams obtainable by laser-generated thermoelastic expansion have been found useful in nondestructive examination of materials. A system utilizing this principle is described in U.S. Pat. No. 4,137,991 to Melcher et al. However, no practical means for scanning and focusing the thermoelastic waves is available except by use of expensive arrays.